
'''Mohs' scale of mineral hardness''' characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. It was created, in 1812, by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. Mohs based the scale on ten readily available minerals. Materials are characterised against the scale by finding the hardest material that they can scratch. The table below shows comparison with absolute hardness measures by a sclerometer. Mohs' is a purely ordinal scale with, for example, corundum being twice as hard as topaz, but diamond, almost four times as hard as corundum.
Some mnemonics traditionally taught to geology students to remember this table are "The Girls Can Flirt And Other Queer Things Can Do" or "To Get Candy From Aunt Fanny, Quit Teasing Cousin Danny".
An alternative table is shown below which has been modified to incorporate additional substances that may fall in between two levels.
'' Source: [http://www.amfed.org/t_mohs.htm American Federation of Mineralogical Societies: Mohs Scale of Mineral Hardness]''
Materials science
Mineralogy
:''Question: if the scale isn't linear is there a linear scale that geologists prefer to use? And what exactly is 'absolute hardness'? How is hardness defined (we know what it is intuitively, but what is its physical definition?) Are there SI units for hardness? -- SJK'' ::There is no absolute hardness scale. No units for hardness either. As for what "hardness" really is, I think that is an open question. It would certainly be related to fracture toughness, but that defies analysis as well, relying on laboratory testing and depending on the geometry of the specimen. :::A simple search on Google shows there is an 'absolute hardness' scale; in fact, there appear to be at least two: the Rosiwal absolute hardness scale; and the Knoop absolute hardness scale. Also, how can a non-existent scale yield real numbers like the ones in this article (although I've more commonly seen 140000 for Diamond, not the 1500 given above; I suppose this might be the Rosiwal vs. Knoop scales.) -- SJK ::::What defines hardness depends on the scale you use. The Rosiwal scale measures resistance to abrasion and uses an arbitrary value of 1000 as the hardness of corundum as its basis (diamond comes out to 140,000). The Knoop scale measures how deep a standard diamond pointed tool can cut into a material given a standard amount of pressure pressure (diamond comes out to 7000 in this scale). :Isn't part of the definition of hardness the scratch test (if you can scratch something with something else, that something else is the harder of the two)? ::Thats the point of the Mohs scale. Frankly, the most useful materials for hardness testing in the field are fingernails (hardness 2-3) and pocket knives (hardness 5-6). Used in conjunction with luster, crystal habit and color, fingernails and pocket knives do a pretty good job. Question: What is the absolute hardness of moissanite? Question: why is "liquid" listed on the second/extended table? It's been there since the table was created, and I cannot see why it isn't "Talc" there. User:194.106.59.2 15:27, 23 Apr 2005 (UTC)
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Mohs scale of mineral hardness
'''Mohs' scale of mineral hardness''' characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. It was created, in 1812, by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. Mohs based the scale on ten readily available minerals. Materials are characterised against the scale by finding the hardest material that they can scratch. The table below shows comparison with absolute hardness measures by a sclerometer. Mohs' is a purely ordinal scale with, for example, corundum being twice as hard as topaz, but diamond, almost four times as hard as corundum.
| Hardness | Mineral | Absolute Hardness |
|---|---|---|
| 1 | Talc (Mg3Si4O10(OH)2) | 1 |
| 2 | Gypsum (CaSO4·2H2O) | 3 |
| 3 | Calcite (CaCO3) | 9 |
| 4 | Fluorite (CaF2) | 21 |
| 5 | Apatite (Ca5(PO4)3(OH-,Cl-,F-)) | 48 |
| 6 | Orthoclase Feldspar (KAlSi3O8) | 72 |
| 7 | Quartz (SiO2) | 100 |
| 8 | Topaz (Al2SiO4(OH-,F-)2) | 200 |
| 9 | Corundum (Al2O3) | 400 |
| 10 | Diamond (C) | 1500 |
| Hardness | Substance or Mineral |
|---|---|
| 1 | Liquid |
| 2 | Gypsum |
| 2.5 to 3 | Gold, Silver |
| 3 | Calcite, Copper penny |
| 4 | Fluorite |
| 4 to 4.5 | Platinum |
| 4 to 5 | Iron |
| 5 | Apatite |
| 6 | Orthoclase |
| 6.5 | Iron pyrite |
| 6 to 7 | Glass, Vitreous pure silica |
| 7 | Quartz |
| 7 and up | Hardenability steel |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Garnet |
| 11 | Fused zirconia |
| 12 | Fused alumina |
| 13 | Silicon carbide |
| 14 | Boron carbide |
| 15 | Diamond |
Mohs scale of mineral hardness
:''Question: if the scale isn't linear is there a linear scale that geologists prefer to use? And what exactly is 'absolute hardness'? How is hardness defined (we know what it is intuitively, but what is its physical definition?) Are there SI units for hardness? -- SJK'' ::There is no absolute hardness scale. No units for hardness either. As for what "hardness" really is, I think that is an open question. It would certainly be related to fracture toughness, but that defies analysis as well, relying on laboratory testing and depending on the geometry of the specimen. :::A simple search on Google shows there is an 'absolute hardness' scale; in fact, there appear to be at least two: the Rosiwal absolute hardness scale; and the Knoop absolute hardness scale. Also, how can a non-existent scale yield real numbers like the ones in this article (although I've more commonly seen 140000 for Diamond, not the 1500 given above; I suppose this might be the Rosiwal vs. Knoop scales.) -- SJK ::::What defines hardness depends on the scale you use. The Rosiwal scale measures resistance to abrasion and uses an arbitrary value of 1000 as the hardness of corundum as its basis (diamond comes out to 140,000). The Knoop scale measures how deep a standard diamond pointed tool can cut into a material given a standard amount of pressure pressure (diamond comes out to 7000 in this scale). :Isn't part of the definition of hardness the scratch test (if you can scratch something with something else, that something else is the harder of the two)? ::Thats the point of the Mohs scale. Frankly, the most useful materials for hardness testing in the field are fingernails (hardness 2-3) and pocket knives (hardness 5-6). Used in conjunction with luster, crystal habit and color, fingernails and pocket knives do a pretty good job. Question: What is the absolute hardness of moissanite? Question: why is "liquid" listed on the second/extended table? It's been there since the table was created, and I cannot see why it isn't "Talc" there. User:194.106.59.2 15:27, 23 Apr 2005 (UTC)
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